[Storage] Small fix for MerkleAccumulatorView::get_consistency_proof
[Storage] Small fix for MerkleAccumulatorView::get_consistency_proof

Since the nodes we are reading are supposed to be frozen and exist in
storage, we should just use `self.reader.get`, so it would return an
error if the node doesn't exist.

Closes: #1096
Approved by: zamsden

aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core